"Crown shyness"

Crown shyness (from Wikipedia) (also canopy disengagement, canopy shyness, or intercrown spacing) is a phenomenon observed in some tree species, in which the crowns of fully stocked trees do not touch each other, forming a canopy with channel-like gaps. 
The phenomenon is most prevalent among trees of the same species, but also occurs between trees of different species. There exist many hypotheses as to why crown shyness is an adaptive behavior, and research suggests that it might inhibit spread of leaf-eating insect larvae.

A prominent hypothesis is that canopy shyness has to do with mutual light sensing by adjacent plants. The photoreceptor-mediated shade avoidance response is a well-documented behavior in a variety of plant species. 
Neighbor detection is thought to be a function of several unique photoreceptors. Plants are able to sense the proximity of neighbors by sensing backscattered far-red (FR) light...

Many species of plant respond to an increase in FR light (and, by extension, encroaching neighbors) by directing growth away from the FR stimulus and by increasing the rate of elongation.

The characterization of these behaviors might suggests that crown shyness is simply the result of mutual shading based on well-understood shade avoidance responses.  A recent study has suggested that Arabidopsis shows different leaf placement strategies when grown amongst kin and unrelated conspecifics, shading dissimilar neighbors and avoiding kin....